I recently did a product shoot for a friend's clothing line. I thought it'd be simple but, for the life of me, couldn't get a fully white background. I used two lights through lightboxes one on the left and one on the right at about 45 degrees. I moved the lights up and down and further and closer away but nothing seemed to help/

Do I need a 3rd light!!! directly above? Challenge was that she wanted the shirts textured so they didn't lay flat. I ended up doing a lot of edit in post to get rid of most of the shadows but any suggestions on lighting this better?

Eliminating the shadows most easily requires lighting the subject and the background seperately.

In your example, suggested methods would be to have the shirt laying on a clear sheet several feet above the BG, using a rear-lit translucent BG, or a combination of the two.

Rear-lit Plexiglas tables are very often used, but if you don't want to invest in a light table you could get something like a Lastolote HiLite that can do double duty as a a portrait BG and as a light table if layed flat.

If you don't do a lot of these kinds of shots and want to avoid a large investment, you could use your current BG and lights, and just get a sheet of tempered glass such as a sliding patio door glass or a glass coffee table top, and support it above your BG with some wood blocks in the corners.

With just two lights, you'd need to have them on low stands so that half the light from each soft box would pass under the shirt to light the BG only, and the other half would be feathered across the surface of the shirt to show shadows and texture. Obviously, more lights would make it easier.

The problem with your lighting setup is that it's basically a copy setup and that's very likely not what you want to show the fabric at its best. You've got the classic double shadow going on - great for copy but not so much for a three dimensional product. A single light and fill will look much nicer. A subtle drop shadow can look great, or if they want the product with no shadow, you can easily knock it out to white. I've shot hundreds of products like this over the years, hell, probably thousands, and even clothes would only take five minutes to draw a path to outline.

As Brian says, this kind of shoot requires independent b/g lighting, especially to reach deeply recessed areas like the armpits. No b/g separation is needed: you could throw the shirt on a SB if the weight is supported. Most refined PP involves the generation of an alpha channel, achieved by two exposures (all lights, then b/g off). The difference of these exposures is alpha which can be used to map an arbitrary b/g, including plain white. This technique can be automated, and will properly render all fabrics, including sheer, lace and furry stuff.